1. Field of the Invention
The present invention concerns a perfected expanding shaft, consisting of an shaft provided with radially mobile elements and with axially mobile elements for the locking of tubular supports coupled with the shaft itself.
2. General State of the Art
It is well known that shafts with expanding elements are very often used to sustain tubular supports on which plastic, aluminium or paper films are wound up or, vice versa, to support the reels when they have to be unwound.
Said expanding shafts, according to a known application, are provided with a plurality of radially mobile sectors, inserted inside slots carried out along the generating lines of the shafts themselves, which buck against the inner surface of the tubular support on/from which the reel is wound/unwound.
The radial exit of the mobile sectors from the relevant slots is generally obtained by means of the thrust due to the inflation of air tubes positioned at the bottom of the slots which house said mobile sectors.
Said expanding shafts are also used to wind up plastic or paper films starting from a single reel which is unwound while supported by another expanding shaft. In this case, as the film is unwound from the reel, it passes through a cutting unit which divides it in strips of the desired length. Each one of said strips is wound on a tubular support coaxially lined with a plurality of further tubular supports, all of which are externally coupled with the same winding expanding shaft.
The skilled persons know that the transversal section of the film which is unwound from the reel doesn't have constant thickness; consequently, not all the supports arranged on the winding expanding shaft, on which the strips obtained from the cutting of the film are wound, have the same diameter at any moment.
Since the winding expanding shaft revolves at constant rpm, the rim speeds of the forming film reels positioned side by side are different; consequently, each film is wound with more or less tension according to said difference in diameter.
In order to eliminate said drawback, upon the increasing of the winding tension each reel should be capable of sliding on the expanding shaft on which it is positioned for the time required to allow the other reels, positioned beside it, to reach the same diameter, so that the tension in each of them is exactly the same.
The expanding shafts of the known type described above offer a solution of the problem which isn't completely satisfying, as the mobile sectors consist of rectilinear metal blocks which are not perfectly suitable for ensuring the friction of the tubular supports on which the film strips are wound.
A further drawback is represented by the fact that during the winding the tubular supports, due to their limited width and also to the winding tension, do not remain rigorously orthogonal to the rotation axis of the expanding shaft, but tend to take an inclined position which compromises both the geometry of the forming rolls and the correct winding tension.